feat: add buildFileUrl utility with tests

Moved from ProjectsSection inline function to shared/lib/utils.
Accepts optional baseUrl for testability without env mocking.

src/shared/lib/index.ts

@@ -1,6 +1,7 @@
 export type { ClassValue } from 'clsx';
 export { CONTACT_LINKS } from './config/config';
 export * from './fonts/fonts';
+export { buildFileUrl } from './utils/buildFileUrl/buildFileUrl';
 export { cn } from './utils/cn/cn';
 export * from './utils/formatDate/formatDate';
 export { groupByKey } from './utils/groupByKey/groupByKey';